This invention relates to a calender in a paper or board machine, particularly a calender comprising at least two soft-faced rolls and at least one hard roll that forms calendering nips with the two soft-faced rolls.
A soft calender in a paper machine usually consists of two nips placed one after the other. Each nip is formed by one hard roll and one soft-faced roll. This type of calender is used in order to facilitate the threading of the web. In an on-line soft calender, it must be possible to thread the web through the nips at full speed. In a calender consisting of two rolls, such threading of the web is possible because the web can run along a linear path. In a calender with three rolls, however, the path of the web is more complicated and the threading is more difficult. Since a soft-faced roll is easily damaged, threading cannot be carried out in the same way as in a calender with hard rolls. In the existing calenders with three rolls, the threading has been carried out while the calender is stationary, or the web has been threaded between the rolls, e.g., by means of air jets or ropes. For the threading, the nips of the calender have been opened. With the prior-art solutions known to the applicants, threading at high web speeds has not been possible at all.